I created a marketing channel segment, but when I break down by marketing channel, other channels appear.

I think this is just an oddity stemming from the use of a calculated dimension (entry page) and a calculated metric (entry)... 

I was getting the same behaviour when I replicated your table in my data, but I can't really explain why...  and given that I know there could be many Marketing Channels per visit, I wouldn't use any "entry" since it wouldn't properly represent multiple channels within the same visit... 

Hi,

For example, if during a 30-day period, the first visit to Page A was through Social, the second visit through Search (page A), and the third visit through Email(page A), shouldn't the marketing channel show only Email in this case?

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No, each hit to the page from each Marketing Channel will be captured and available. The previously set values are not overwritten.

You are correct that there is a "most recent" attribution, but that means that the currently set Channel persists for 30 days (or if you changed the retention period, to your custom set time) or until it's overwritten. But overwriting the value only affects that hit and forward. There are also rules about not overwriting specific channels (i.e. Social, Search, Email, etc) with Direct or Internal channel (since these would almost certainly overwrite on each instance, basically killing your 30 day attribution).

So:

• Social Media Site Click
• Page A
  • Marketing Channel = "social"
  • Marketing Channel Instance triggered (MCI for short in these examples)
• Page B
  • "social" channel is retained
  • No MCI
• User leaves and comes back a week later
• Page C
  • Despite the user directly entering the site, direct will not override the specific "social" channel, which is still set
  • No MCI
• User comes back two months later
• Page D
  • Marketing Channel has expired, so "Direct" will now be recorded
  • MCI is triggered
• Page E
  • "direct" channel is retained
  • No MCI
• User gets an email, that links back to Page D, user clicks on it
  • Marketing Channel is set to "email"
  • MCI is triggered

If you look at Page D, you will see both "direct" (first hit on the page) and "email" (second hit on the page).

Now Let's add a few more events to the Page D scenario:

• Paid Ad on Social
• Page D
  • Marketing Channel set to "Paid Social" will now be recorded
  • MCI is triggered
• Page E
  • "Paid Social" channel is retained
  • No MCI
• Shop Page
  •  "Paid Social" channel is retained
  • No MCI
  • User makes a purchase (Order metric is triggered)
• User gets an email, that links back to Page D, user clicks on it
  • Marketing Channel is set to "Email"
  • MCI is triggered
• Shop Page
  
  •  "Email" channel is retained
  • No MCI
  • User makes a second purchase (Order metric is triggered)
• Page D
  •  "Email" channel is retained
  • No MCI

If you look at your orders by Marketing Channel, again, you will see one for "Paid Social" and one for "Email". IF that first hit to Page D (and all previous 30 day hits were overwritten) you would lose your attribution to you entry page, all navigated pages, and most importantly those purchases which were driven by a paid campaign... this could lead the marketing team to think their campaign was a failure.

It would also be incorrect to say that both purchases were driven by an email... since the user only received the email after making the first purchase.

Now, you might be saying, okay... but if the user comes back a third time (within the 30 days) directly, I would still be tracking "email" as the channel... yes, that's correct... but again, email is a specific driver, whereas direct doesn't have a lot of "juice" behind it.

However, if you changed the Marketing Channel to "Direct" on that third visit, your last real campaign will never last the 30 days... every new visit to the site would overwrite it... so you would really be making it visit attribution.

Also, just to clarify, is a marketing channel instance not a single visit value, but rather a metric that gets recorded multiple times?

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The Marketing Channel Instance is a specific metric that is tied to when the Marketing Channel is set.

If you set a Marketing Channel, and it persists for 30 days, every single page view, occurrence, visit, etc will be associated to that Marketing Channel (the value is applied to every single tracking hit)... the Instance is specific to only the page that fit the Marketing Channel rule (i.e. had a campaign string, came from a specific referrer, etc).

And since Marketing Channels can be set multiple times in the same visit, and the entry page and entries pertains to the first hit of the visit, you will be missing channels (also, since both of those are calculated, they seem to have some odd behaviours when you use them with Marketing Channels... without digging really deep into all the data, I am not sure why its doing what it's doing... but since there are easier ways to pull the data, that also better reflect what is happening... it's probably best to go that route.

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I also have another question! In the example you mentioned, since each page has different sources of traffic, it makes sense that Social, Search, and Email would all be included. But when looking only at Page A, should the marketing channel show just Social as the single value?

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Yeah, in my first example, I only visited Page A once, and it was set to "Social", so yes, in that isolated example you should only see Social... but in my later examples, let's look at Page D, the second version using Paid Social and Email:

You will notice that Page D had 3 page views in total, but only 2 of those page views actually came from a Marketing Channel... so while you will see 3 pvs with marketing channel (1 social, 2 emails), you will only see 2 Marketing Channel Instances (1 social, 1 email) as those are the only two hits that actually set a Marketing Channel. All of this took place inside a single visit.